The present invention relates to a method for preventing the arching of glass sheets in the roller-equipped furnace of a horizontal tempering plant, in which method the glass sheets are carried in a horizontal plane by means of a conveyor consisting of horizontal rollers through a furnace, the opposite surfaces of a glass sheet being subjected to a conduction, convection and radiation heat effect generated by resistance elements, rollers or the like furnace components fitted above and below a glass sheet whereby, in order to equalize the total thermal effect applied to the top and bottom surfaces of a glass sheet, the top surface is subjected at least at the initial stage of a heating cycle to an intensified convection heat effect by blasting air into the furnace adjacent to the top surface of a glass sheet as narrow jets which through an injector effect produce a turbulent flow for the hot air contained in the furnace along the top surface of a glass sheet.
The invention relates also to an apparatus for preventing the arching of glass sheets in the roller-equipped furnace of a horizontal tempering plant, said apparatus comprising a furnace, heating resistances therein for maintaining the furnace temperature close to a glass softening temperature, horizontal rollers inside the furnace for supporting a horizontal glass sheet and providing a conveyor therefor, as well as a blasting manifold above a bearing surface formed by the rollers for blowing the heat-equalizing air into the furnace.
This type of method and apparatus are disclosed in the Applicants' U.S. Pat. No. 4,390,359. This method has proved highly effective for eliminating a serious problem, namely that at various stages of heating the top and bottom surfaces of a glass sheet are subjected to different heat effects which, at the initial stage of heating, tend to arch a glass sheet in a manner that the edges rise upwards and at the final stage of heating the direction is reversed. This is particularly due to the fact that a cold glass sheet advances into a furnace and onto hot rollers which initially deliver heat rapidly to the bottom surface of a glass sheet but towards the end of a heating cycle the temperature of a glass sheet approaches the temperature of rollers to increase the proportional share of the amount of heat delivered to the top surface of a glass sheet. Thus, this problem has been substantially alleviated by means of a heat-equalizing blast known from U.S. Pat. No. 4,390,359.
It has been discovered, however, that this prior known method and apparatus has not in every case provided a sufficient heat transfer above the glass. Particularly, at the very beginning of a heating cycle, a more effective compensation should be accomplished to the heating between bottom and top surfaces.